Giant crater clue to extinction disaster

The scar of a huge comet or asteroid impact believed to have triggered the largest extinction event in history has been discovered off the north-west coast of Australia.

The possible cause of the "Great Dying", which was much earlier and more extensive than the one that wiped out the dinosaurs, has been found by a team led by a scientist at the University of California, Santa Barbara.

The research suggests that vast numbers of species die off when a huge body hits the Earth, not only throwing up dust and debris that block the sunlight, but also triggering intense volcanic activity.

Most scientists agree that a meteor impact in Chicxulub, in the area of Mexico's Yucatan Peninsula, accompanied the extinction of the dinosaurs 65 million years ago, though the picture is confused by massive volcanic activity that occurred at around the same time in the Deccan Traps of India.

Today in the journal Science, Dr Luann Becker of the University of California, Santa Barbara, reports extensive evidence for a 125-mile-wide crater called the "Bedout High" which dates from the time period known as the end-Permian, when the Earth was configured as one vast land mass called Pangea and a super ocean called Panthalassa.

The team found meteoric fragments in a thin claystone "breccia" layer. They also found "shocked quartz" in this area and in Australia.

Dr Becker commented that there are few earthly circumstances that have the power to disfigure quartz, which is an extremely stable mineral even at high temperatures and pressures deep inside the Earth's crust.

The team made the discovery after Dr Becker found out that oil companies in the early 70s and 80s had drilled two cores into the Bedout structure in search of oil and gas.

She and co-author Robert Poreda went to Australia to examine these cores held by the Geological Survey for Australia in Canberra. "The moment we saw the cores we thought it looked like an impact breccia," said Dr Becker. In the cores, the team found evidence for a melt layer formed by an impact.

The primary marine and terrestrial victims included trilobites, corals, fish, plankton and sabre-toothed dog-like gorgonopsid reptiles and their rhino-sized prey. Other groups that were substantially reduced included shellfish, sharks, bony fish, ostracodes and echinoderms.